quantification of compaction effects on soil physical properties and crop growth

www.elsevier.com/locate/geoderma

Geoderma 116 (2003) 107–136

Quantification of compaction effects on soil physical

properties and crop growth

J. Lipieca,*, R. Hatanob

a Institute of Agrophysics, Polish Academy of Sciences, P.O. Box 201, 20-290 Lublin, PolandbBioscience and Chemistry, Faculty of Agriculture, Hokkaido University, Sapporo 060, Japan

Abstract

A quantitative description of soil compaction effects is required to improve soil management for

reducing compaction problems in crop production and environment. Our objective is to provide a

review of indices and methods used to quantify the effects of compaction on soil physical properties

and crop growth. The paper starts with the description of available methods to quantify stress and

displacement under traffic. The following few sections deal with methods and parameters used to

characterise the effect of compaction on soil strength, oxygen, water, heat and structural arrangement

with consideration of spatial variability. The effect of soil compaction on macroporosity and

associated water movement, aeration and root growth is discussed. One section is devoted to

integrated systems to measure simultaneously more than one soil physical property. Potential of some

advanced developments in computer-assisted tomography (CAT) and nuclear magnetic resonance

(NMR) for non-destructive 3D quantification of soil structure, roots and root water uptake as affected

by soil compaction is indicated. Finally, some techniques useful for quantifying root and shoot

growth, and water uptake in relation to soil compaction are discussed. The models available allow

assessment of compaction effects on some behavioural soil properties based on the inherent properties

and bulk density of soil. Additional research is required on the effect of compaction on soil structural

discontinuities that substantially affect many soil functions and root growth in the whole profile.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Soil compaction; Physical properties; Root and shoot growth; Measurements

1. Introduction

Compaction of agricultural soils is an increasingly challenging worldwide problem for

crop production and environment (Van Ouwerkerk and Soane, 1994; Soane and van

Ouwerkerk, 1995). The vast majority of soil compaction and shearing in modern

0016-7061/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0016-7061(03)00097-1

* Corresponding author.

J. Lipiec, R. Hatano / Geoderma 116 (2003) 107–136108

agriculture is due to vehicular traffic, which is an integral part of the soil management

system. Increasing size of agricultural implements is a significant cause of induced soil

compaction and deterioration of soil structure. In addition, many agronomic practices have

to be performed frequently in a very short period of time and when soil is wet and

conducive to compaction. This results in deeper stress penetration and subsoil compaction

(Van den Akker and Stuiver, 1989).

Alterations in soil structure due to compaction influence many aspects of the soil such

as strength, gas, water and heat, which in turn affect root and shoot growth and

consequently crop production and environmental quality. Proper quantification of soil

compaction effects is essential to develop management strategies that minimise the

harmful compactive effect. In this paper we review the indices and measurement

approaches which are relevant to the quantification of the behavioural soil physical

properties and crop growth in response to compaction. Response of the indices to soil

compaction in relation to soil type and experimental conditions is discussed.

2. Measuring stresses and strains

The methods to measure stress and strains (displacements) including the theory

were thoroughly reviewed by Horn and Baumgartl (1999). The use of relatively large

size of measuring devices causes considerable disturbance in soil structure and

therefore recent developments tend to miniaturise sensors and measure stress and

displacement simultaneously (Kuhner et al., 1994; Trautner and Arvidsson, 2000;

Tarkiewicz and Lipiec, 2000; Pytka and Konstankiewicz, 2002). In the system

described by Tarkiewicz and Lipiec (2000), stress is measured using silicone oil

container covered by rubber membrane fitted with pressure transducer and displace-

ment by recording the optical fibre positioning laser sensor using CCD camera. The

size of measuring head being inserted into the soil was reduced to 25� 10� 5 mm.

The advantage of the system is that the data can be collected in time intervals of

tenths of a second, which are characteristic for stresses acting upon running wheels

(Or and Ghezzehei, 2002). The system allows precise detection of changes of stress

and displacements at various soil depths under laboratory and field conditions. An

important factor of soil displacement is shearing, which together with compaction

affect soil deformation (Horn et al., 1998).

The measuring systems revealed that stress and displacement are strongly affected by

loading, water content and soil type. For example, the stress in a swelling/shrinking clay

loam with an axle load of 14 mg varied from 300 to 650 kPa at 0.3 m depth and from 75 to

270 kPa at 0.7 m depth depending on the soil water content (Trautner and Arvidsson,

2000). The soil displacement under heavy sugar beet harvesters (35 mg) was recorded at

0.3 m depth of dry sandy clay loam and up to down 0.7 m depth at field water capacity

(Arvidsson et al., 2001). Repeated wheeling at constant water content causes a relative

increase in the vertical principal stress compared with horizontal stress components (Horn

et al., 1995). In case of tractors, rear wheels cause higher stress than front wheels

(Weisskopf et al., 2000). At the same stress applied the stress and strain formation

decreases with increasing soil aggregation and is smaller under conservation than

J. Lipiec, R. Hatano / Geoderma 116 (2003) 107–136 109

conventional tillage due to higher internal soil strength in the former (Horn and Baumgartl,

1999; Horn and Rostek, 2000).

The data of stress and strains are required for proper quantification of structural

dynamics in a soil profile under external forces by mathematical upscaling (Horn et al.,

1998; Or and Ghezzehei, 2002).

3. Indices of the state of soil compactness

Dry bulk density and total porosity are commonly used to characterise the state of soil

compactness. However, these properties have a limited value for comparison of the state of

compaction between soil types. To overcome this problem, actual bulk density is

expressed as a percentage of some reference compaction state of given soil and called

degree of compactness or relative compactness.

The degree of compactness proposed by Hakansson (1990) is defined as the ratio of the

actual bulk density to the reference bulk density obtained by uniaxial compression of wet

soil (sufficiently for drainage) at a static pressure of 200 kPa. Fig. 1 shows that the critical

limits of penetration resistance and air-filled porosity were similarly related to the degree

of compactness in soils different in texture, porosity and water holding properties

(Hakansson and Lipiec, 2000). The measurements of penetration resistance of variously

compacted soils in this study were carried out during the growing season at a range of soil

water contents and air-filled porosity was calculated from the water contents and

porosities. It is worth to note that the derived lines for the matric water potentials of

� 1500 and � 10 kPa were crossed by the critical penetration resistance (3 MPa) and air-

filled porosity (10%) at very similar and close to optimal values (86–88) of the degree of

compactness for crop production. There was not such similarity between the soils when

using dry bulk density.

Fig. 1. Critical limits of penetration resistance (upper line) and air-filled porosity (lower line) as functions of the

degree of compactness and matric water potential in the plough layer in a loamy sand (1), a light loam (2), a silty

loam (3) and a clay loam (4) (after Lipiec and Hakansson, 2000).


Another relative compaction value is the ratio of actual bulk density and maximum bulk

density obtained in the Proctor compaction test, in which a constant energy (by a falling

weight) is applied at different water contents. This ratio has been useful to characterize soil

compaction in field studies (Pidgeon and Soane, 1977; Carter, 1990; Da Silva et al., 1994).

The Proctor test is recommended for homogenized soil material in studies of the effect of

water content and soil composition on soil compactability (Horn and Lebert, 1994).

In the approach of Bennie (1991), the relative compaction index was defined as:

½ðqactual � qminÞ=ðqmax � qminÞ�;

where q is the bulk density; the maximum bulk density (qmax) is determined with the Proctor

test; and qmin is the minimum bulk density—from themass of unsieved dry soil needed to fill

a container of known volume. Values of the index < 0.5, 0.5–0.6, 0.6–0.7 and >0.7

correspond to low, medium, high and very high degrees of compaction, respectively.

The relative compaction parameters are more useful than bulk density or porosity in

studies of the effects of field traffic on soil conditions and root and crop response

(Canarache, 1991; Hakansson and Lipiec, 2000). Using the relative compaction instead of

bulk density enhanced the performance and applicability of least limiting water range

(LLWR) by reducing differences in its values between different soil types (Da Silva et al.,

1997). The LLWR indicates soil water content range at which the effect of water potential,

aeration and mechanical impedance on root and shoot growth is minimal (Da Silva et al.,

1994). The relative compaction was also suitable input parameter in modelling response of

root growth, leaf area and crop yield to soil compactness in various soil–climate

combinations (Arvidsson and Hakansson, 1991; Simota et al., 2000).

Monnier et al. (1973) proposed another concept based on structural and textural

porosity. The structural porosity, i.e. the pore space related to biological activity and the

arrangement of clods and cracks and thus to compaction, was distinguished from the

textural pore space between the elementary particles. The structural pore space together

with morphological analysis of soil profile was useful for describing the soil volume

affected by vehicular traffic (Richard et al., 1999). A decrease in structural porosity

resulted in an increase in relict structural pores being remnants of structural pores distorted

during traffic and accessible only through the necks of textural (lacunar) pores (Bruand et

al., 1997; Richard et al., 2001). Backscattered electron scanning images (BESI) and

mercury porosimetry showed this. The volume of the relict structural pores is indicative of

soil compaction and its effects on behavioural properties (Richard et al., 2001).

In civil engineering, void ratio (ratio of volume of voids and volume of solids), being

independent of particle density, is widely used to measure degree of compaction.

4. Quantification of compaction effects

4.1. Soil physical characteristics

For accurate assessment of changes in soil fabric due to compaction, measurements of

bulk density are not adequate (Dexter, 1997; Horn and Rostek, 2000; McQueen and

Shepherd, 2002) and should include other soil properties. Measurements of soil strength,


aeration, water, thermal and structural characteristics are identified as the main behavioural

properties influencing the quality of the soil after compaction. Changes in the character-

istics with time provide information on the sustainability of the soil.

4.1.1. Soil strength

Soil strength measurements, such as cone resistance, shear strength, aggregate strength

and precompression stress, are widely used to assess soil structure following compaction

(Guerif, 1994; Horn and Rostek, 2000). Cone resistance is most frequently used to assess

soil compaction for correlation with root growth, draft requirements and soil structure.

Penetrometers used differ in diameter and angle of cones. Most field penetrometers have

cone diameters from 11 to 25 mm and semi-angles of 15j or 30j (Ehlers et al., 1983;

Campbell and O’Sullivan, 1991; ASAE, 1993).

Since measurements of cone resistance are relatively rapid they are suitable in detecting

strength and structural discontinuities associated with wheel tracks and size of structural

units (Lowery and Morrison, 2002). Vertical discontinuity usually occurs between

aggregated seedbed and compacted soil below after seed bed preparation or between

tilled layer and untilled subsoil (Glinski and Lipiec, 1990; Smucker and Aiken, 1992).

Kozicz (1996) reported that penetration resistance of the subsoil was more than twice

greater than that in plough layer that was compacted at harvesting of winter wheat and 10

times greater than in plough layer prepared for sowing. Over the whole profile, highly

compacted zones and their spatial irregularities in the layer affected the spatial arrange-

ment of roots in the non-compacted subsoil (shadow effect) and thus availability of water

reserves (Tardieu, 1988). Strength variations of soil may also occur in laboratory experi-

ments where uniform and highly reproducible soil conditions are needed (Koolen and

Kuipers, 1983). They result mostly from different moisture homogenization and clod

prevention and air explosion.

Campbell and O’Sullivan (1991) indicate that cone resistance effects due to wheeling

should be evaluated as soon as possible after the passage of wheels, because changes in

matric potential and hydraulic conductivity will change water content below the wheel

track. According to ISO (ISO, 1998; Whalley et al., 2000), at water status greater than field

capacity mechanical impedance should be determined by large cone penetrometers, which

allow detection of small mechanical impedance in wet soil, and between field capacity and

wilting point—by small cone penetrometers.

In structured soils, if the maximum diameter of the cone is smaller or larger than the

structural units, the penetrometer resistance is mostly a function of intraaggregate strength

and interaggregate strength, respectively (Bradford, 1986; Lowery and Morrison, 2002).

Sharp penetrometer cones of diameter similar to that of roots are recommended when

mechanical impedance is characterised with respect to rooting (Groenevelt et al., 1984;

Voorhees et al., 1975; Whalley et al., 2000). Voorhees et al. (1975) reported that root

growth was better correlated with soil resistance to 5j than 30j semi-angle conical probe

since a failure of soil by sharp penetrometer is similar to that by roots. Root growth in this

study was also more closely correlated with normal soil resistance (which does not include

soil–metal friction) than to total point resistance (which includes the friction component).

Setting appropriate critical values is of importance for good simulation of root growth

(Dexter, 1987; Stenitzer, 1988; Lipiec et al., 2003). In the modelling work of Stenitzer


(1988), the critical values of cone resistance that cause the beginning of reduction of root

growth ranged from 1 to 1.7 MPa and those stopping root growth ranged from 3 to 4 MPa,

depending on soil type and pore size distribution, especially for cone diameter greater than

that of roots.

Although penetration resistance is regarded as a useful measure of soil impedance to

root growth (Bengough and Mullins, 1990), it has some limitations. One of these

drawbacks is a relatively great spatial variation because this property is a point measure-

ment rather than a bulk soil measurement. Thus, a great number of measurements is

required for precise estimation. Geostatistical analysis showed higher spatial dependence

(as characterised by range of the semivariogram) of penetration resistance (at a meso scale)

in loose soil than in compacted soil (Perfect et al., 1990; Lipiec and Usowicz, 1997). This

implies that sampling interval for representative results should be smaller in loose soil. To

strengthen the value of point measurements of cone resistance further, studies to develop

relations between cone resistance and not-point measurements of shear strength are needed

(Glinski and Lipiec, 1990). Lowery and Morrison (in press) indicated that current and new

developments will make the penetrometry technology more reliable with respect to the

assessment of soil variability and site-specific farming because the procedures are

relatively rapid and provide limited invasive action.

To characterize the mechanical strength of aggregates in relation to the effect of soil

management and compaction, measurements of small cone penetrometer resistance

(Perfect et al., 1990; Becher, 2000) and tensile strength (Lipiec and Tarkiewicz, 1986;

Guerif, 1994) are used. It was shown that compacted zones of cultivated soil are

characterized by the higher values of strength parameters (Becher, 2000; Munkholm et

al., 2002) and a greater percentage of large aggregates (Voorhees, 1983; Bakken et al.,

1987; Guerif, 1994). Becher (2000), using a 0.55-mm probe for aggregates 10–15 mm,

observed considerable scattering with depth of penetration and between the replications.

He attributed this to the penetration of macropores, touching or pushing sand grains, very

small dense microaggregates and varying bulk density among the aggregates. Despite the

great variability, the different strength behaviour of soil aggregates at matric potentials

lower than field capacity (more negative) can be related to soil management.

Study in a range of aggregate diameter (from 9 to 30 mm) showed that aggregate tensile

strength decreases with increasing moisture and increases with increasing bulk density due

to traffic (Lipiec and Tarkiewicz, 1986). Irrespective of the level of soil compaction and

water content, the greatest crushing strength characterizes the smallest aggregates.

Measurements of the scaling of specific volume, pore size distribution, and pore scaling

suggest that the increased strength is due to the reduction in crack sizes available for

fracture (Hallett et al., 2000). Horn (1990) indicates that the strength of single aggregates,

determined as the angle of internal friction and cohesion, depends on the number of

contact points and the forces, which can be transmitted at each single contact point.

Mechanical strength of aggregated soil compared to structureless, homogenised soil

material is increased and leads to reduced soil compressibility and compactability (Horn

et al., 1998; Horn and Baumgartl, 1999).

Another indicator of strength and mechanical stability of soil is precompression stress

(Horn and Fleige, 2000; Arvidsson et al., 2001; Berli, 2001). Berli (2001) reported a linear

relationship between the precompression stress and initial water content (negative) and


logarithms of negative soil water potential (positive). The effect of soil moisture was much

stronger in the subsoil than in the toposoil. Horn and Fleige (2000) developed regression

equations to calculate the precompression stress by using values of the cohesion and the

angle of the internal friction. The equations take into account soil texture, structure and

moisture, and therefore they can be used for various soil types under different water

conditions. Generally, the precompaction stress at a given pore water pressure increases

with increasing soil aggregation. At the same stress applied, the stress and strain formation

is smaller in soil under conservation than conventional tillage system (Horn and Rostek,

2000). Calculation of the soil physical properties after exceeding precompression stress

provides information about plant growth conditions and is useful tool for recommenda-

tions for sustainable land use (Horn and Fleige, 2000).

4.1.1.1. Effect of plant roots. Root diameters are usually larger than most soil pores and

soil particles are pushed aside and the bulk density of the soil near the root increases

(Dexter, 1987; Bruand et al., 1996). The dense fabric of the soil around roots affects many

physical, chemical and biological aspects (Glinski and Lipiec, 1990).

Scanning electron microscopy is used for quantitatively examining variations in the

micro- and meso-porosity within the soil around the roots (Fig. 2) by image analysis.

Bruand et al. (1996) reported that maize root reduced porosity by 22–24% and increased

bulk density up to 1.80 mg m� 3 close by the root–soil interface, although it was 1.54 mg

m� 3 in the bulk soil. The modelling work of Dexter (1987) indicated that the soil density

around roots decreases exponentially with distance from the root surface with an exponent,

which is a constant multiple of the root diameter. This can be enhanced in compacted soil

where roots are typically shorter and thicker (Lipiec and Simota, 1994).

To non-destructively monitor changes in spatial distribution of bulk density and water

content close to the root, dual-source g-CT scanning was used with satisfactory precision

(Phogat et al., 1991; Asseng et al., 2000). Relatively long scanning limits efficient use of

the technique. Another useful application of tomography involves the use of NMR to

determine heterogeneous water uptake around a single root (Young, 1998; Young et al.,

2001) and 3D visualisation of roots from 2D matrices (Asseng et al., 2000).

Fig. 2. Backscattered electron scanning image (BESI) showing the dense fabric immediately around the maize

root. Pores occupied by resin are black, silt particles are light grey, and porous clayey phase is dark grey (Bruand

et al., 1996). B.d.: bulk density.


4.1.2. Aeration

Soil compaction effects on soil aeration are usually quantified by air-filled porosity,

oxygen diffusion rate (ODR), redox potential and air permeability (Stepniewski et al.,

1994).

Air-filled porosity is most often used to evaluate soil aeration, and the value of < 10%

(v/v) is regarded as critical for plant growth. However, at a similar air-filled porosity, the

equivalent pore diameter can be much smaller in compacted than in uncompacted soil

(Simojoki et al., 1991). This may result in different air permeability, which is directly

related to the square of the diameter of the air-filled pores (Stepniewski et al., 1994). Thus,

it seems that transmission parameters and contribution of active pores better reflect

aeration status of compacted soil.

The response of air permeability, being a measure of the ability to transport gas by

convection, to compaction is related to soil structure and pore size and pore continuity. At the

same level of compactness, air permeability was greater for coarse structure (4–8 mm peds)

compared to fine structure (< 2 mm peds) (Lipiec, 1992). Ball et al. (1994) reported that in

long-term experiments (20–25 years) on imperfectly drained loams, air permeability was

lower under direct drilled than conventionally ploughed soil. However, in another study

(Schjønning and Rasmussen, 2000), the pore system in sandy and sandy loam soils was well

connected regardless whether the soils were ploughed or directly drilled, whereas on the silt

loam ploughing introduced a limiting permeability, which was slightly eliminated by 4-year

direct drilling. Due to the close dependence of air permeability on pore diameter, the

measurements show high variability and require high replication (Koszinski et al., 1995;

Gysi et al., 1999; Iversen et al., 2001). Air permeability of 1.0� 10� 12 m2 (as measured by a

constant flux air permeameter at � 5 kPa water potential) was suggested as the critical lower

limit for agronomic performance of poorly drained soils (McQueen and Shepherd, 2002).

However, high air permeability is associated with low precompression stress (Horn and

Rostek, 2000) and indicative of presence of interaggregated pores vulnerable to compaction

(Lipiec, 1992; Stepniewski et al., 1994; Gysi et al., 1999).

The ratio of air permeability and macropore volume (Blackwell et al., 1990; Carter et

al., 1994) or air permeability and air-filled porosity (Groenevelt et al., 1984; Ball et al.,

1994) is considered as a measure of pore continuity and pore organisation. Lower values

of the ratio well reflected reduced pore continuity due to compaction (Lipiec and Glinski,

1997; Munkholm et al., 2002).

Relative gas diffusion coefficient (D/Do), being a ratio of the gas diffusion coefficient in

soil (D) and the diffusion coefficient of the same gas in atmospheric air (Do), is indicative

of continuity and tortuosity of the pores. This coefficient decreases with increasing soil

compactness, particularly in wet soil (McAfee et al., 1989, Stepniewski, 1981; Stepniew-

ski et al., 1994).

The rate of oxygen supply from the soil air to the roots can be characterised by the

oxygen diffusion rate (ODR) (Stepniewski et al., 1994; Dexter and Czyz, 2000; Whalley et

al., 2000). The ODR is measured with a platinum microelectrode, on which oxygen is

reduced, thereby simulating its uptake by the root. The electrodes should be inserted

immediately before each measurement because of precipitation carbonates and hydroxides

that impair measurements. This is difficult in compacted soil and can be largely

diminished by strengthening the electrode casing (Czyz, 1989). The measurements of


ODR are limited to wet soil (0 to � 100 kPa) when the electrode is covered with a water

film (Glinski and Stepniewski, 1985).

The effect of increasing soil compaction and wetness on the decrease of ODR is clearly

illustrated in literature (Stepniewski et al., 1994; Dexter and Czyz, 2000). This decrease is

against greater oxygen consumption per unit of root grown in compacted soil (Glinski and

Lipiec, 1990). An ODR of approximately 25 Ag m� 2 s� 1 is generally regarded as the

lower limit, below which root growth is negligible.

Another indicator of soil aeration is redox potential which is useful to characterise

reduction processes in very wet soils (close to or at saturation) and anoxic conditions,

where oxygen flux measurements are of less value (Stepniewski et al., 1994; Dexter, 1997;

Whalley et al., 2000). The electrodes for measuring redox potential in contrast to those for

ODR can be left in situ for longer periods and still provide good results.

4.1.3. Hydraulic properties

4.1.3.1. Water retention curves. Some studies indicated that an increase in soil compac-

tion results in lower gravimetric water content at high matric potential range (from 0 to

approximately � 16 kPa) and higher—at low values of the potentials (from� 50 to � 1550

kPa) (Walczak, 1977; DomzalC, 1983). Only a slight effect occurred at the intermediate

potential range. However, volumetric water content at high matric potentials range (from 0

to � 10 kPa) diminished with increasing soil compaction and slightly increased with at the

range of low potentials (from � 250 to � 1550 kPa) (Walczak, 1977; DomzalC, 1983;

Kutılek and Nielsen, 1994; Ferrero and Lipiec, 2000). These are reflected in flattening of soil

water retention curve (SWRC) and they are indicators that as the proportion of large pores

decreases, the proportion of small pores increases (Assouline et al., 1997; Van Dijck and Van

Asch, 2002). As reported by Assouline et al. (1997) for matric potential � 100 MPa, the

volumetric water content in the compacted soils is somewhat lower and can be attributed to

the reduced potential of surfaces. However, in the � 100 and � 1500MPa range, very small

pores of compacted fabric retain more water and its films are absorbed to particle surfaces.

Direct measurements of SWRC are time-consuming and expensive and to overcome this,

limitation pedotransfer functions (PTF) are being developed to predict the SWRC frommore

easily measurable and more readily available particle-size distribution, organic matter and

bulk density. A reasonable estimation of SWRC for soil bulk density changes due to tillage

provides simple empirical models proposed by Ahuja et al. (1998). Rajkai et al. (1996),

using pedotransfer function, obtained the best prediction of SWRC when fitted cumulative

particle-size data, together with the clay and silt fractions, and the bulk density were used.

Pachepsky et al. (1998) indicated that including penetration resistance as a parameter related

to soil structure in pedotransfer functions improves the accuracy of estimating SWRC from

soil texture and bulk density. In Europe, available hydraulic data of more than 5000 soil

horizons from 12 countries were brought together into one central database HYPRES

(Hydraulic Properties of European Soils) and pedotransfer functions were derived (Wosten,

2000). Standardization of the data was achieved by fitting the Mualem–van Genuchten

model parameters to water retention h(h) and hydraulic conductivity K(h). Cornelis et al.

(2001), evaluating nine pedotransfer functions, concluded that most PTFs predict moisture

content well near saturation and permanent wilting point. The former is mainly dependent on


total porosity and the latter on bulk density and clay content. The highest prediction errors

were at moisture conditions close to field capacity, due to mostly different morphology of

pore volume. Pachepsky et al. (1996) and Koekkoek and Booltink (1999) used neural

networks (NNs) to estimate soil water retention. The NNs performed slightly better than the

regression-based PTFs in both works. The performance of both NNs and regressions was

comparable when van Genuchten’s equation was fitted to data for each sample, and the

parameters of this equation were obtained from texture and bulk density. Changes in

volumetric water contents at given potentials affect the hydraulic conductivity.

4.1.3.2. Saturated flow. Saturated hydraulic conductivity (Ksat) is often used to charac-

terise the effect of soil compaction on water flow. A drastic reduction of Ksat with

increasing compaction has been reported in many studies (Dawidowski and Koolen, 1987;

Debicki et al., 1993; Hakansson and Medvedev, 1995). The ratio of Ksat or water

infiltration rate of loose and compacted soil range from several (Young and Voorhees,

1982) to several hundreds (Horton et al., 1994; Arvidsson, 1997; Guerif et al., 2001).

A reduced Ksat will enhance runoff and soil erosion (Young and Voorhees, 1982; Fleige

and Horn, 2000). The critical limit for adequate Ksat (as measured with a constant head

method) for poorly drained fine-textured soils in cropping systems was established at

1.0� 10� 6 m s� 1 (McQueen and Shepherd, 2002). However, in highly permeable and

conducive-to-leaching sandy soils, reducedKsat conductivity may improve their water status

(Agraval, 1991; Sharma et al., 1995; Lipiec et al., 1999) and reduce NO3–N leaching losses

(Agraval, 1991).

The effect of soil compaction on saturated water flow is largely governed by larger

pores (preferential flow) (Ehlers, 1975; Lin et al., 1996, 1999; Lipiec et al., 1998), which

are negatively related to soil compaction (Carter, 1990). In the experiment with stained

water (Lipiec et al., 1998; Hakansson and Lipiec, 2000), increasing soil compactness

induced by vehicular traffic reduced volume of stained pores ( =macropores that actively

contributed to the water flow) than the volume of all macropores (>30 Am). As can be seen

in Fig. 3, a relatively greater reduction in stained area and number of stained pores with

Fig. 3. Percent of stained areal porosity relative to total area and number of stained pores in horizontal sections

(0.036 m2) in the silty loam at various tractor-wheel traffic (after Lipiec and Hakansson, 2000).


increasing traffic intensity occurred in the plough layer than in the subsoil where vertically

oriented earthworm channels dominate. Research indicates that compaction may reduce

not only the volume of macropores but also their continuity. This corresponds with other

results showing that under conservation tillage the presence of continuous larger pores

increase saturated hydraulic conductivity despite a higher bulk density (Lipiec and

Stepniewski, 1995; Arvidsson, 1997).

The active macropores have a significant effect on the water flow. Lin et al. (1996)

reported that 10% of macropores (>0.5 mm) and mesopores (0.06–0.5 mm) contributed

about 89% of the total water flux. As shown by Ehlers (1975), the maximum infiltrability

of conducting channels in the stronger untilled soil was more than 1 mm/min, although the

volume of these channels amounted to only 0.2 vol.%. According to Poiseuille’s law,

water flow rate in the tubular pores is proportional to the square of the pore diameter.

Reduced infiltration due to compaction leads to lower wetting of main root zone (Kulli et

al., 2000). Preferential flow is frequently the dominant mechanism for water flux in subsoil

layers that contain biopores and planar voids (Horton et al.,1994).

Structure of the channels and their functions can be an effective measure of soil

‘quality’ as they are relatively resistant to vertical compression (Alakukku, 1996). Lin et

al. (1999) proposed to incorporate macroporosity as a criterion of soil structure in a soil

morphological system. However, under specific direction and extent of stresses by

wheeling (‘‘dynamic loading’’) on wet soil, total porosity and macroporosity may increase

but their transmitting functions are limited because of poor continuity (Weisskopf et al.,

2000). In the study of Munkholm et al. (2002), macroporosity (>30 Am) was negatively

correlated with tensile strength of soil.

In their review, Horton et al. (1994) indicate that pressure infiltrometers are particularly

useful to quantify the hydraulic conductivity response to soil compaction both in situ or in

the laboratory on undisturbed samples. However, the so-called tension infiltrometers

measure a very shallow depth of soil (5–7 cm) and thus do not measure macropore

continuity, hence hydraulic conductivity, with depth in the soil profile. Saturated hydraulic

conductivity of the compacted soil can be computed based on the parameters of water

retention curves and inherent properties and bulk density of soil using regression models

(Mualem, 1986; Assouline et al., 1997; Guerif et al., 2001). Incorporating the macropore

flow component into models that assume a horizontally homogeneous soil profile improves

their performance in predicting water distribution and chemical movement in soil profile

(Walczak et al., 1996; Ludwig et al., 1999; Kumar et al., 1999; Borah and Kalita, 1999).

4.1.3.3. Unsaturated flow. Unsaturated flow largely affects the dynamic processes of

water and solute movement in the vadose zone. Experimental data relating the effect of

soil compaction on unsaturated flow is very limited. It has been reported (Walczak et al.,

1993; Horton et al., 1994; Guerif et al., 2001; Richard et al., 2001) that hydraulic

conductivity, as a function of soil wetness, generally decreases with compaction; however,

at some compaction range and low water potentials, the conductivity is higher in

compacted versus non-compacted soil. Analysis of the relations between hydraulic

conductivity and water ratio indicates the effect of soil compaction on hydraulic

conductivity by increasing the contact surface between aggregates and by formation of

the relict structural pores that do not contribute to water movement (Richard et al., 2001).


An important factor that affects the absorption of rainfalls is sorptivity of aggregates

(Leeds-Harrison et al., 1994; Dexter, 1997). A miniaturised disc permeameter described by

Leeds-Harrison (1994) has been useful to measure the aggregate sorptivity which was

lower for aggregates from compacted than uncompacted soil (Lipiec et al., 2002). Horn et

al. (1994) reported that, in more compacted single aggregates compared with bulk soil, the

unsaturated hydraulic conductivity at low negative pore water pressure range (from 0 to

� 800 hPa) is decreased. However, the inverse was true at very negative pore water

pressure values (Hillel, 1980, quoted by Horn et al., 1994). If rainfall is faster than can be

absorbed by the aggregates at the soil surface, the excess runs down the inter-aggregate

pores (Dexter, 1997).

Tension discs and pressure ring infiltrometers are usually used for the in situ estimates

of the characteristic just below saturation. The techniques are particularly suitable for

surface soils that are strongly affected by compaction. The results of Angulo-Jaramillo et

al. (2000) provide a simple and fast means of measuring the infiltration rate and

determining volume of the macropores allowing fast transfer of both water and solutes

and preferential flow parameters at near saturation state. Infiltration measurements with

solutions of 18O and Cl� as tracers are a promising tool for the determination of mobile/

immobile water content fraction.

The effect of soil compaction on unsaturated hydraulic conductivity in undisturbed soil

cores can be well characterised using the instantaneous profiles of moisture and matric

potentials in the tensiometric range (Walczak et al., 1993).

The unsaturated hydraulic conductivity of compacted soil can be defined by the

Mualem–van Genuchten model based on soil-saturated hydraulic conductivity and

parameter accounting for the correlation between pores and the flow path tortuosity

(Mualem, 1976; Van Genuchten, 1980; Assouline et al., 1997). The approach of Assouline

et al. (1997) can be applied to both drying and wetting retention curves and thus

determines the effect of compaction on the hysteresis domain.

The changes in hydraulic conductivity are used in models for simulating water and

chemical movement and redistribution in soil profile. Recent reviews indicate (Walczak et

al., 1997; Lipiec et al., 2003) that most of the models are based on the Darcy/Richards

one-dimensional flow, and some of them have potential to quantify compaction effects.

Bulk density (or total porosity) mostly represents soil compactness in the models.

Unsaturated hydraulic conductivity, together with root length density, is the main factor

affecting hydraulic resistance in unsaturated compacted soil (Lipiec and Tarkiewicz,

1988). Further work is needed to develop modelling approaches with consideration of soil

structural discontinuities and spatial variation of the input parameters resulting from

compaction.

4.1.4. Heat transport

High thermal conductivity and heat capacity characterise solid and water phases in

contrast to air phase of soil. Therefore, any soil management practice affecting soil

compactness and thus relative proportion of each phase will have an effect on the thermal

properties and propagation of heat (Usowicz et al., 1996).

As can be seen in Fig. 4, the thermal conductivity, heat capacity and thermal diffusivity

(ratio of the thermal conductivity and volume heat capacity) increase with increasing soil

http:\\www.ipan.lublin.pl\~usowicz\

Fig. 4. Thermal properties and coefficient of variation (CV) of loamy sand as affected by tractor passes (after

Usowicz et al., 1995). The approach of Usowicz (1992) used for the calculation of the properties is available on:

http://www.ipan.lublin.pl/-usowicz/.


compaction to higher extent in wetter soil. Coefficient of variation (CV) values indicate

lower variability of the properties in compacted than uncompacted soil. Abu-Hamdeh

(2000), Abu-Hamdeh and Reeder (2000) and Guerif et al. (2001) reported a similar effect

of compaction on thermal properties. Increase in soil thermal properties with compaction is

attributed to mostly improved contact between soil particles. Horn (1994) reported that

greater thermal conductivity and heat conductivity in aggregated soil in a wide range of

soil water content depend not only on the continuity of contact points (conductance) but

also on the continuity of water-filled pores (convection and diffusion). The study of Turk

et al. (1991) indicated that soil with aggregated structure compared to disturbed soil is

characterised by greater heat flow irrespective of bulk density. Abu-Hamdeh (2000)

reported that using a single-probe device consisting of a heater and a temperature sensor is

a good way of obtaining temperature-by-time data in the field to determine thermal

conductivity. This method reflected the responses of the thermal conductivity to varying

soil bulk density under different tillage treatments well.

Usowicz et al. (1996) reported that the spatial variability of thermal properties over the

cultivated field was lower in compacted than loose soil. Bulk density and water content are

the main factors affecting this variability. The effect of soil bulk density on thermal

conductivity was more pronounced at high (field capacity or greater) than medium soil

water contents, as shown by spatial autocorrelation. This research also showed that the

spatial variability of thermal diffusivity is determined by bulk density rather than by soil

water content. The use of quick TDR measurements of soil water content facilitates the

study of spatial distribution of soil thermal properties, which requires numerous measure-

ments (Malicki, 1990; Usowicz et al., 1996).

http:\\www.ipan.lublin.pl\~usowicz\


Alterations in the thermal properties due to compaction affect the soil temperature and

its temporal and spatial variability. The effect of compaction was reflected in the lower rate

of warming and cooling, the daily temperature fluctuations and the values of the noon

temperature in the topsoil (Lipiec et al., 1991; Boone and Veen, 1994). Soil with high

thermal conductivity compared to low thermal conductivity can exhibit lower surface

temperature amplitudes under equal heat flux densities (Abu-Hamdeh, 2000). At greater

depths, however, a higher temperature was noted in compacted soil. The differences can be

attributed to greater volumetric heat capacity and thermal conductivity in compacted soil at

similar soil water content (Lipiec et al., 1991). Relatively large wetness and associated

evaporation from the compacted soil (Nassar and Horton, 1999) will enhance this effect on

topsoil temperature.

When soil temperature decreases with depth, a commonly deeper root system in loose

soil may experience a lower temperature than a shallow root system in compacted soil.

4.1.5. Structural arrangement

Measurements of pore space are increasingly used to quantify the effects of soil

compaction on the soil structure (SlCowinska-Jurkiewicz and DomzalC, 1991; Douglas and

Koppi, 1997; Lipiec et al., 1998; Richard et al., 1999; Pagliai et al., 2000). Morphological

analysis of pore space in the field using the geometry of half-ellipse was used to evaluate

relative percentage of the compacted zones (massive zones without visible macropores)

produced by wheel tracks (Richard et al., 1999; Boizard et al., 2002). This analysis

revealed that the percentage of compacted zones of loamy soil increased at the soil

moisture >0.15–0.16 g g� 1 during harvesting and sowing, and at >0.21 g g� 1 during

seedbed preparation when the lowest machinery load is applied (Richard et al., 1999).

To evaluate the soil compaction effects on pore and aggregate structure, images of

resin-impregnated soil are used (SlCowinska-Jurkiewicz and DomzalC, 1991; Horn et al.,

1995; Lipiec et al., 1998). Morphological analysis of the images revealed that compaction

of loamy soil by tractor pass reduced larger pores, but mainly the elongated and

continuous transmission pores (50–500 Am) and to lesser extent those < 50 Am (Pagliai

et al., 2000). Transmission pores were reduced more by rubber-tracked tractor than by

wheeled tractor, and this was reflected in lower infiltration (Pagliai et al., 2000; Servadio et

al., 2001). In sandy soils, reduced infiltration due to compaction was attributed to packing

pores corresponding to the fabric of the elementary particles (Coulon and Bruand, 1989).

Use of the resin-impregnated soil is expensive, time-consuming and requires speci-

alized training. This limits the availability of the methods. In response to the call for easily

available techniques, Holden (1994) presented a fast and inexpensive method for

quantifying soil macropores in soil blocks by using water-soluble impregnate and house-

hold paint. This technique can be useful in relating pores >0.3 mm to preferential flow,

aeration and root development in relation to soil compaction.

The pore structure patterns of variously compacted soil exhibit fractal self-similarity

and can be described via the box-counting, two-dimensional fractal dimension D2 (Hatano

et al., 1992; Lipiec et al., 1998). The D2 values indicated decreasing space-filling

behaviour with increasing soil compaction. The fractal technique was useful to evaluate

both distribution of pore area and roughness of pore outline in freshly tilled and

consolidated soil (Gimenez et al., 1997). As indicated by Oleschko et al. (1997), fractal


dimensions are close to the entropy values of the soil structural system and therefore can

be an indicator of soil physical degradation that is an issue of current concern. The fractal

parameters of soil pore surfaces can characterise soil degradation due to loss of organic

matter and intensive cyclic wetting–drying under different management systems (Pachep-

sky et al., 1995). Numerous uses of fractals in quantification of soil structure are discussed

in detail by Perfect and Kay (1995), Baveye et al. (1998) and Young et al. (2001).

The above quoted results indicate that compaction decreases the diversity of structure of

larger pores and makes the soil less heterogeneous. However, the diversity of soil structure

at the cluster or grain scales may increase in compacted soil due to shearing effect

(Warkentin, 2000). The increased diversity of grain surfaces leads to exposing fresh surface

for adsorption of organic compounds. The surfaces play an important role in the storage and

release of water and nutrients, and provide habitat for soil microorganisms (Horn and

Lebert, 1994; Warkentin, 2000). The response of pores less than 50 Am to compaction (or

management) is quantified by mercury intrusion porosimetry (Pagliai et al., 2000).

New CCD cameras and high-resolution scanners (< 10 Am) are being developed; they

offer potential for more detailed quantification of soil structure and solute and water

movement in relation to soil compactness. (Young et al., 2001; Gantzer and Anderson,

2002). However, their prevalent use is now delineated by sample size, costs and require-

ment of computer power owing to large files. A 2-Am resolution image analysis of 1 cm3

sample requires approximately a terabyte of data (Young et al., 2001).

Recent developments in computer-assisted tomography (CAT) scanning based on

generating transmission images (Perret et al., 1999) and detecting nuclear radiation

emitted from the soil (single photon emission computed tomography—SPECT) (Perret

et al., 2000; Young et al., 2001) can be used in the 3D quantification of macropore network

(set of interconnected macropores) at the scale of soil core or column. Olsen and Børresen

(1997) pointed out that the nondestructive CT was a very useful tool when data on

macroporosity and bulk density are needed prior to other investigations, e.g. distribution of

infiltrating water. Analysis of the macropore networks at different water contents provides

insight into the hydraulic behaviour of soil.

4.1.6. Combined measurements

Few integrated systems with the capability of evaluating more than one soil property

affected by soil compaction are available. Such systems minimise disturbance of soil.

To monitor the changes in the spatial distribution of both bulk density and water

content, low- and high-energy sources for CT scanning have been used (Phogat et al.,

1991; Aylmore, 1993; Rogasik et al., 1999). In the approach of Rogasik et al. (1999), X-

ray computed tomography is determined for energy levels of 80 and 120 kV, using scanner

continuously rotating fan beam-measuring system. The data obtained enable analysts to

calculate the distributions of dry bulk density, water, air and solids in undisturbed soil core

samples with a resolution of 0.25 mm in horizontal direction and 1 mm in vertical

direction. The authors indicate that existing scanning systems with resolutions of 2–10 Amcan be useful for specific investigations, such as macropore wall roughness, density

distribution within aggregates and soil–root relations. However, increasing resolution-

scanning system will use the smaller sample with respect to labour intensity and handling

of data.


In other approaches, penetrometers are equipped with TDR probe sensors for measur-

ing volumetric soil water content (Young et al., 2000; Vaz and Hopmans, 2001; Vaz et al.,

2001) or sensor of molecular polarisation (Mobitech, 1998). TDR probe (two parallel

conductor and ground steel wires) can be embedded into a cone penetrometer (Morrison et

al., 2000), wrapped around the insulation-covered penetrometer rod (Vaz and Hopmans,

2001) or penetrometer cone (Vaz et al., 2001). The design with TDR probe on the

penetrometer cone compared to that on the penetrometer rod has increased sensitivity due

to improved soil–TDR contact (Vaz et al., 2001). Morrison’s device is mounted on a

hydraulic-powered truck probe that is capable of penetrating to a depth of up 1.5 m

(Morrison et al., 2000; Lowery and Morrison, 2002).

Combined measurements system (Malicki et al., 1992; Walczak et al., 1993; Whalley et

al., 1994) of TDR and tensiometry provide data on soil water content and matric potential.

The system allows for frequent readings of instantaneous profiles of soil water content and

matric potential during transition from saturated to air-dry state in undisturbed soil cores to

determine soil water characteristics and unsaturated hydraulic conductivity. The usefulness

of the system in characterising the compaction effects on the hydraulic properties has been

reported by Walczak et al. (1993).

The main benefits of combined measurements are that they minimise disturbance of soil

and are performed within the same soil volume at the same spatial location, thus

preventing complications due to soil heterogeneity.

4.2. Crop growth

4.2.1. Roots

A common response of root system to increasing compaction level is decreased root

size, retarded root penetration and smaller rooting depth (Glinski and Lipiec, 1990). This is

mostly due to excessive mechanical impedance and insufficient aeration depending on soil

wetness. Decreased root size results in greater distances between the neighbouring roots

and affects water and nutrient uptake (Tardieu, 1988; Glinski and Lipiec, 1990; Yamaguchi

and Tanaka, 1989). Table 1 shows that the half distance between the nearest maize roots on

horizontal planes within the depth of 30 cm is below 5.8 mm for uncompacted soil and

Table 1

Mean (1986–1988) half distances (mm) between the neighbouring roots at the heading growth phase of spring

barley

Layer (cm) Silty loam Loamy sand

Tractor passes Tractor passes

0 1 3 8 0 1 3 8

0–10 1.4 1.2 1.1 0.9 1.3 1.4 1.1 0.9

10–20 3.2 3.4 6.4 1.27 2.5 3.0 4.5 9.1

20–30 5.8 6.4 10.6 6.37 3.7 4.9 31.8 63.7

30–40 0.6 31.8 – – 15.9 21.2 – –

40–50 15.9 – – – 63.7 – – –

50–60 31.8 – – – 63.7 – – –

Calculated using the data of root length density (L) in Lipiec et al. (1991) and the formula: 1/(pL)1/2.


increases up to 63.7 mm at depth 20–30 cm in the most compacted soil. However,

absorption of water and nutrients usually takes place in the soil adjacent to the root surface

from 2 to 8 mm, depending on soil and nutrient types (Yamaguchi and Tanaka, 1989). This

leads to reduced water and nutrient uptake and crop yield (Section 4.4).

There is a range of methods that could be used to quantify root response to compaction

(Atkinson and Mackie-Dawson, 1991; Atkinson, 2000; Smit et al., 2000). Since root

measurements are expensive and time-consuming and there is no universal method

suitable for all situations, the concept ‘fit for purpose,’ assuming that measurements are

designed for specific needs including model involving root parameters, is being widely

used (Atkinson, 2000). Recent developments based on computer-assisted tomography

(CAT) and magnetic resonance imaging (MRI) provide potential to non-destructive

measurements of spatial distribution of bulk density and the dynamics of plant root

systems and water uptake (Asseng et al., 2000).

The presence of macropores of a diameter greater than the roots is an important

structural discontinuity which affects root growth. A soil matrix with macropores will offer

greater potential for undisturbed root growth because the roots can bypass the zones of

high mechanical impedance (Ehlers et al., 1983; Tardieu and Manichon, 1986; Hatano et

al., 1988; Glinski and Lipiec, 1990). Fig. 5 illustrates similar distribution patterns of

macropores and roots. The percentage of roots grown into existing pores and channels

increases in deeper and stronger layers (Goss, 1991) where they can be the only possible

pathways for root growth. The preferential root growth into macropores will lead to

increasing critical limits of soil compactness (Etana et al., 1999; Hakansson and Lipiec,

Fig. 5. Distribution patterns of macropores (MP) and roots (RT) of maize: (A) pot experiment (after Hatano et al.,

1988); (B) field experiment (after Tardieu and Manichon, 1986).


2000) and soil strength measured by large penetrometer cones (Ehlers et al., 1983,) which

are relatively insensitive to pores having diameter greater than the roots and to hetero-

geneity in the soil at the scale of root tip (Bengough, 1991; Whalley et al., 2000). In

coarse-textured soils, high mechanical impedance can result from sand particles inter-

locking to resist displacement (Panayiotopoulos, 1989; Glinski and Lipiec, 1990).

The effect of soil compaction on root growth was also less inhibited in coarse-

structured soil (aggregates 4–8 mm) compared to fine-structured (structural units < 2

mm) soil although penetration resistance was higher in the former (Busscher and Lipiec,

1993). This implies the presence of pores larger than growing roots.

The larger pores are also beneficial in poorly aerated soils since they drain at higher

water potential (less negative) and remain air-filled for longer periods compared to smaller

pores. This results in decreasing critical values of air-filled porosity (Boone et al., 1986;

McAfee et al., 1989) although part of the soil matrix can be anoxic (Zausig et al., 1993;

Hakansson and Lipiec, 2000). McQeen and Shepherd (2002) suggested the critical lower

limit set of macropore volume at 5 m3 100 m� 3 for cropped sites on poorly drained soil.

Scarcity of experimental data on root and crop response to the soil heterogeneity is a

limiting constraint in modelling work (Lipiec et al., 2003).

The relationship between the distribution of macropores and roots can be numerically

described via fractal analysis (Hatano and Sakuma, 1990; Lipiec et al., 1998). Fig. 6 shows

that the trend of fractal dimension values with depth for flow-active pores (Ds2) and roots

(Dr2) were similar in both loose and moderately compacted soil. This is indicative of the

relationship between distribution patterns of the pores and roots.

The higher values of Dr2 in the upper soil appear to reflect greater number of roots,

which can be due to greater branching in loose soil (Eghball et al., 1993) and characteristic

superficial root growth in compacted soil (Glinski and Lipiec, 1990).

4.2.2. Water and nutrient uptake

Reduced and unevenly distributed roots in compacted soil affect uptake rate (per unit of

root) and total uptake of water and nutrients. Increased water uptake rate in compacted soil

was reported for bean (Huang, quoted by Smucker and Aiken, 1992), maize (Veen et al.,

1992; Lipiec et al., 1993), barley (Lipiec et al., 1992) and rice (Glinski and Lipiec, 1990).

This increase was mostly attributed to a greater root–soil contact and to a higher

unsaturated hydraulic conductivity and a greater water movement towards the roots.

The increased root water uptake rate of Kentucky Bluegrass in poorly aerated compacted

soil was linked to higher root porosity and thus increased root permeability (Agnew and

Carrow, 1985). In most experiments, however, increased water uptake rate was not

sufficient to compensate entirely for the reduction in total root length and resulted in

reduced total water uptake. Similarly, greater nutrient inflow rate per unit length and root

soil contact area without additional nutrient application were not sufficient to compensate

for reduced root size (Veen et al., 1992; Lipiec and Stepniewski, 1995). Uneven root water

uptake is of great importance in the modelling of water and nutrient use by plants and

redistribution in the soil profile (De Willigen and van Noordwijk, 1987; Schmidhalter et al.,

1994; Novak, 1995; Walczak et al., 1997). To precisely quantify the water uptake from

variously compacted layers, a procedure based on the analysis of the data of chloride (as a

tracer) uptake by plants and its diffusion was useful (Lipiec et al., 1993).

Fig. 6. Fractal dimensions for internal structure of methylene blue stained pores (Ds2) and root distribution patterns

(Dr2) in loose (L) and medium compacted (MC) silty loam. SD: standard deviation (after Lipiec et al., 1998).


Approaches with split root systems in soil of varying bulk density and matric potential

are useful in studying the effect of spatial distribution of mechanical impedance and

aeration on root growth and function (Bar-Yosef and Lambert, 1981; Gowing et al., 1990;

Stepniewski et al., 1994; Lipiec et al., 2001). The studies revealed that reduced root

growth and water uptake in strong or anoxic sites can be partly compensated for in

favourable local environments. The extent of this compensatory response depends on the

severity of compaction and water status. Column experiments with variously compacted

soil layers provide the opportunity to separate depth effects from strength effects (Busscher

et al., 2000). The separate the effect of subsoil compaction on root growth and uptake,

functions can be quantified in the field by removing topsoil for time of compaction (Ishaq

et al., 2001). The methods for control and measurement of the physical environment in

root growth experiments are reviewed at length by Whalley et al. (2000).

4.3. Stomata diffusive resistance

Root systems grown in compacted soil are often subjected to wetting and drying which

influence the stomata functioning. An experimental system using water-filled ceramic tubes

under controlled pressure below atmospheric for controlling soil water potentials (over the

tensiometric range) has been found to be useful for studying stomata behaviour in response

to varying water status in variously compacted soil (Lipiec et al., 1996). Fig. 7 shows that

with transient wetting, the stomata resistance and its variation over the growth period were

considerably higher in a severely compacted soil than in low or medium compacted soil. A

substantial increase of stomata resistance in most compacted soil occurred when soil matric

potential increased from � 415 to � 220 hPa due to poor aeration. The highest stomata

diffusive resistance in most compacted soil has also been reported in droughty period


Fig. 7. Stomata resistance in maize and soil matric water potential as a function of days after planting (after Lipiec

et al. 1996). B.d.: bulk density, LSD: least significant difference.


(Lipiec and Glinski, 1997). Ali et al. (1999) reported that the increased leaf stomata

resistance occurred even before a measurable change in leaf water potential.

4.4. Crop yield

Crop yields in compacted soils are mostly associated with the extent and function of

the root system. The yield decrease in overcompacted soil is frequently attributed to

excessive mechanical impedance (Lipiec et al., 1991), reduced water infiltration and crop

water use efficiency (Radford et al., 2001), insufficient aeration (Czyz and Tomaszewska,

1993) or their combination depending on weather conditions. Under relatively dry

conditions, soil compaction at planting time increased upward water movement and the

final crop yield tended to be less reduced in wet seasons and in seasons with favourable

rainfall distribution than in dry seasons (Lipiec and Simota, 1994). The combination of

mechanical impedance and rainfall for selected periods during growing season in

regression leads to improved description of yield variations associated with soil manage-

ment and compaction (Kossowski et al., 1991; Busscher et al., 2001). This interactive

effect is particularly important in predicting crop yield of sandy soils where strength

problems are enhanced by low available soil water content and velocity of the soil water

movement down the soil profile.

Mathematical modelling contributes to better understanding of the complex and

variable effects. In most models available, root growth is predicted as a function of

mechanical impedance and water status of soil and crop yield—from interactions of soil

water and plant transpiration and assimilation (Lipiec et al., 2003).

5. Concluding remarks

This paper reviews the indices and methods used to quantify the effects of soil

compaction on strength, air, water, heat and root and shoot growth. A wide range of the

indices and methods is used. The selection process (which of them should be used)

depends on soil type, climate and severity of compaction. A few measuring systems for

simultaneous measurement of two or more soil physical characteristics, which minimise

soil disturbance and prevent complications due to soil heterogeneity, are available.

Examples include systems for simultaneous measuring stress and displacement or

penetrometer resistance and soil water content.

Geostatistical analysis was useful to show lower spatial variability of some character-

istics (e.g., penetrometer resistance, thermal properties, and soil structure) at meso scale in

compacted than loose soil. This implies a need for smaller sampling interval in the latter.

However, the diversity of soil structure at the field scale or cluster and grain scales may

increase in trafficked soil due to the distribution of wheel tracks and shearing effect,

respectively.

Macroporosity, being inversely proportional to soil compaction, has a significant effect

on water and solute flow and root growth. There is a considerable potential to improve

modelling work by incorporating macroporosity as an input data. The relationship between

the distribution of macropores and roots can be described by fractal analysis.


Recent developments in CAT and NMR scanning provide major tools for non-

destructive 3D quantify soil macropores (networks, tortuosity, connectivity) and roots at

small scale, using reconstruction techniques from 2D matrices. The innovative techniques

show definite promise for future quantification of various properties or processes on the

same site or sample. However, their wide use at the moment is limited by the requirements

of large amounts of time, specific equipment and computer power.

Laboratory approaches with split root systems between soil of varying bulk density and

matric potential are useful to study the effect of spatial distribution of mechanical

impedance and aeration on root growth and function. They revealed that reduced root

growth and water uptake in compacted soil can be partly compensated for in favourable

local environments. The extent of this compensatory response depends on the severity of

compaction and water status. Column experiments with compacted layers and weakly

compacted soil gives the opportunity to separate depth effects from strength effects.

Further work is needed to study the soil compaction effects on structural discontinuities

which strongly affect water, gas, solute transport, nutrient cycling, root growth, and the

better use of small-scale data at the larger scale.

6. Uncited references

Okhitin et al., 1991

Van den Akker and Carsjens, 1989

Van den Akker and Stuiver, 1989

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